When a complete cut of the spinal cord is made at the middle thoracic level in the neonatal rat at the 7th postnatal day (PN7) or earlier, the hindlimbs spontaneously recover the ability to make rhythmic steps, enabling the animal to walk, but with an ataxic gate. Recovery in minimal to absent when the cordotomy is done at PN14 or later. The results of H-reflex testing have demonstrated the Gpla fibres make a strong functional contact with alpha motoneurons (alpha-MNs) in spinalized PN7 rats, but not in untreated PN14 rats. The alpha-MNs in the PN14 animals fire continually at 5-7 Hz, but do not respond, or respond very weakly to Gpla stimulation. The receptive field for alpha- MNs of the triceps surae muscles also becomes greatly enlarged in spinalized PN7 rats. Treatment of the spinalized, PN14 rats with GM1 ganglioside, or injection of an E14 ventral mesencephalic (VM) cell suspension that contains 20% tyrosine hydroxylase positive (TH+) cells into the lumbosacral spinal cord, causes a degree of functional recovery in the spinalized PN14 rat that is statistically equivalent to the spontaneous recovery seen in the untreated PN7, spinalized rat. The lumbosacral central pattern generator (CPG) therefore displays a high degree of plasticity and is clearly capable of profound functional modification. There is a large body of neurophysiologic results detailing the modification of the lumbosacral CPG by catecholamines. The transplantation of fetal dopaminergic neurons into the lumbosacral spinal cord of the spinalized rat, has produced prolonged functional recovery in the spinalized PN14 rat. effectively duplicating the transient effects observed after the acute in vivo administration of catecholamines or their agonists. A dense reinnervation of the lumbosacral spinal cord with TH+ varicosities is observed after injection of the VM cell suspension into the spinal cord of the PN14 rat, although neuronal survival is poor. A similar phenomenon has also been observed in vitro, in which greater than 95% of the plated cells die by DIV14. However, the TH+ cells that survive elaborate enormous dendritic arborizations. There is also a selective survival of TH+ cells in the culture after DIV14, when greater than 60% of the surviving neurons are TH+. Our preliminary evidence suggests that a neurotrophic factor (NTF) is elaborated specifically by astrocytes of the ventral mesencephalon, and rescues the TH+ cells that survive after DIV12 from death, and promotes elaborate dendritic growth. We intend to isolate and molecularly characterize of the NTF that appears to be specific for rescuing dopaminergic neurons from death. We will use this NTF to attempt to enhance the survival of dopaminergic neurons in vivo after transplantation, and study the effects produced on functional recovery.